The oceans play a key role in regulating the Earth's climate. But how does the warming of the oceans due to climate change affect precipitation and storms? A study now shows that warming of the upper water layers in the Indo-Pacific strengthens the East Asian monsoon. As the sea releases more heat and water vapor into the atmosphere, the atmosphere becomes wetter and can trigger more frequent and heavier rainfall. This has an impact on the regional and global hydroclimate.
The Indo-Pacific Warm Basin (IPWP), spanning the western Pacific and eastern Indian Oceans, is the most extensive body of warm water on Earth. With surface temperatures above 28 degrees Celsius year-round, the IPWP releases large amounts of water vapor and heat into the atmosphere, contributing to the development of tropical storms with heavy rainfall. The Asian monsoon and its often heavy rainfall are also influenced by this warm zone.
look in the past
"The Indo-Pacific thermal basin acts like a steam and heat engine for the global climate," explains a team led by Zhimin Jian from Tongji University in Shanghai. "The heat content of its upper water layers plays a particularly important role in the energy flow and hydrological cycles of the earth's surface." It is therefore all the more important to understand the changes in the IPWP and their possible effects. Measurement data show that this Indo-Pacific warm zone has expanded rapidly in recent decades as a result of man-made climate change and the associated warming of the oceans. At the same time, the frequency and intensity of tropical storms increased.
"However, modern observations can only record temporary reactions," the scientists explain. "Thus, for a deeper understanding, it is crucial to look back in time and determine the relationships between equatorial Pacific water temperature fluctuations and the tropical hydroclimate using paleoclimate records." To achieve this, Jian and his reconstructed team measured the temperature of the upper water layers of the Indo-Pacific Warm Basin down to a depth of 200 meters for the last 360,000 years. They were helped by the remains of calcareous marine organisms, which store different amounts of calcium and magnesium depending on the water temperature. The researchers combined the information obtained from this with climate models and compared them with data on the strength of the East Asian monsoon over the same period.
Ocean heat influences monsoons
The result: Just as today's short-term observations suggest, an increased heat content of the Indo-Pacific Warm Basin also led to heavier monsoon rains in East Asia in the past 360,000 years. "Our study suggests that changes in the thermal structure of the western Pacific control the release of moisture, latent heat, and precipitation over East Asia," summarizes co-author Mahyar Mohtadi from MARUM - Center for Marine Environmental Sciences at the University of Bremen. "The temperature gradient between different latitudes controls not only energy uptake from the tropical Pacific, but also how winds carry moisture from the ocean to land."
The data show that the warming of the ocean at that time and thus also the strength of the monsoon were mainly determined by the angle of incidence of the solar radiation. Thus, both sea temperatures and monsoon strength have fluctuated in concert with shifts in the Earth's orbit, which occur approximately every 23,000 years and change the distribution of incident solar radiation at each latitude.
Important for global climate regulation
"In our simulations, which account for the rotational behavior of the Earth, an increase in upper water temperatures of the IPWP is accompanied by a release of latent heat and an increase in precipitation rate," the authors report. "This means that more oceanic heat is transferred to the monsoonal continents and released for further poleward atmospheric heat transport." The water temperatures in the Indo-Pacific Warm Basin and the East Asian monsoon are thus also crucially involved in the regulation of the global hydroclimate. From the authors' point of view, the findings can help to forecast future climate changes more precisely.
Source: Zhimin Jian (Tongji University, Shanghai, China) et al., Nature, doi: 10.1038/s41586-022-05302-y